The present invention relates to a thermopolymerizable composition for obtaining a highly ion-conductive solid polymer electrolyte, a solid polymer electrolyte obtained by polymerizing the thermopolymerizable composition, a battery and an electric double-layer capacitor using the solid polymer electrolyte, and processes for manufacturing the same.
In view of the trend of downsizing and desire to eliminate non-solid components in the field of ionics, demand has increased for commercial use of solid-state primary or secondary batteries and electric double-layer capacitors using solid electrolytes as a new ionic conductor rather than the conventional electrolytic solutions.
More specifically, in conventional batteries with electrolytic solutions, leakage of the electrolyte solution or elution of the electrode substance outside the battery is likely to occur, which presents a problem in long-term reliability.
Electric double-layer capacitors using a carbon material having a large specific surface area as the polarizable electrodes and disposing an ionic conducting solution therebetween also have problems in long-term use and reliability because as long as the capacitor uses an existing electrolytic solution, leakage of the solution outside the capacitor is likely to occur during the use for a long period of time or when a high voltage is applied. Electric double-layer capacitors using conventional inorganic ionic conducting substances additionally have a problem that the decomposition voltage of the ionic conducting substances is low and the output voltage is low.
On the other hand, batteries and electric double-layer capacitors using a solid polymer electrolyte are free of problems such as leakage of the solution or elution of the electrode substance and can be processed into various forms or easily be sealed. Also, these can be easily reduced in the thickness. Furthermore, it is reported that in the case of an electric double-layer capacitor using a polyphosphagen-based organic polymer as the main component of ionic conducting substance, the output voltage is high as compared with those using an inorganic ionic conducting substance (see, for example, JP-A-4-253771 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)).
Studies of solid polymer electrolytes in general have succeeded in improving the ionic conductivity to approximately from 10xe2x88x924 to 10xe2x88x925 S/cm at room temperature, however, this is still low by more than two orders of magnitude as compared with that of liquid ionic conducting substances. The same applies to solid polymer electrolytes having introduced thereinto an oligo-oxyethylene chain, which are being taken notice of in recent years (see, for example, U.S. Pat. No. 5,194,490). Furthermore, there is a problem that at low temperatures of 0xc2x0 C. or less, the ionic conductivity generally decreases to an extreme extent.
For installing a solid polymer electrolyte into a battery or electric double-layer capacitor, a method of using an electrolyte and a polymerizable compound as the main components of solid polymer electrolyte, loading these into a battery or capacitor structure body in the form of liquid or gel and compounding them by curing, is being studied.
For curing the polymerizable composition, curing methods using active rays are heretofore being aggressively studied and investigated. However, in view of the construction of battery, it is difficult to simultaneously compound and integrate respective elements of positive electrode, negative electrode and/or separator, with the polymerizable composition for solid polymer electrolyte by the irradiation of light. Particularly, in the case where a positive electrode, a solid polymer electrolyte and a negative electrode are stacked or rolled up, each element does not transmit light and therefore, they are difficult to integrate. Furthermore, the polymerizable composition is prone to oxygen inhibition by the contacting air and this disadvantageously gives rise to curing failure.
A method by heat curing is also proposed, in which respective elements of positive electrode, negative electrode and/or separator, and the solid polymer electrolyte may be cured and at the same time, compounded and integrated, and the internal impedance of the battery can be reduced. This method is superior for a type having difficulties in the curing by active rays, that is, a type where a positive electrode, a solid polymer electrolyte and a negative electrode are stacked or rolled up. However, in the case of a polymerizable composition for solid polymer electrolyte using a thermopolymerization initiator, the initiator is mostly determined by the desired curing temperature, therefore, for example, when the electrolytic solution contains a low boiling point solvent, use of initiators having radical generation at high temperatures is limited so as not to cause changes in the solution composition due to volatilization of the solvent. To cope with this problem, a polymerization accelerator is used in combination so that curing at a temperature of from room temperature to a medium temperature (about 80xc2x0 C.) can be performed. The polymerization accelerator or decomposition products thereof, however, deteriorates the current properties such as ionic conductivity of the solid polymer electrolyte or the properties such as cycle life. If the curing is performed only by heating without using any polymerization accelerator, due to dependency of the curing rate on the thermal decomposition rate of the thermopolymerization initiator, it takes a long time to reach the curing when the temperature is low. It is a common technique to increase the amount of the polymerization initiator or radicals generated so as to efficiently perform curing. However, the amount of unreacted initiator or decomposition products increases, and these disadvantageously have adverse effects on the current properties such as ionic conductivity or electrochemical properties such as cyclability.
As such, if the curability of the thermopolymerizable composition is increased by using an initiator or accelerator having high initiation efficient at low or medium temperatures or by increasing the amount of polymerization initiator, there arises a problem in the storage stability such as gelation of the thermopolymerizable composition, increase in viscosity, etc. accordingly, a polymerizable composition for solid polymer electrolyte, having good heat curability and excellent storage stability is being keenly demanded.
An object of the present invention is to provide a thermopolymerizable composition for a solid polymer electrolyte having excellent ionic conductivity at room temperature and also at low temperatures and having sufficient strength, said thermopolymerizable composition having good curability, excellent storage stability and high practicability in which a polymerization initiator having good thermopolymerization initiating ability and a polymerizable compound having good curability are combined and a specific polymerization retarder is used to prolong the duration of possible use of the thermopolymerizable composition.
Also, an object of the present invention is to provide a solid polymer electrolyte having high ionic conductivity and good stability, comprising an electrolyte and a polymer having a cross-linked and/or side chain structure obtained from the above-described thermopolymerizable composition.
Another object of the present invention is to provide a primary battery and a secondary battery, capable of working at high capacity and high current, having long life and high reliability, and being produced at a low cost, in which the above-described solid polymer electrolyte is used inside the battery.
Still another object of the present invention is to provide an electric double-layer capacitor with a high output voltage, a large takeout current, good processability, long life, excellent reliability and profitability in the production, in which the above-described solid polymer electrolyte is used inside the capacitor.
As a result of extensive investigations to attain the above-described objects, the present inventors have found that the compound having a specific structure represented by the following formula (1) has a polymerization retarding effect and thereby improves storage stability of a thermopolymerizable composition: 
(wherein the symbols have the same meanings as defined later).
Furthermore, by combining a thermopolymerization initiator comprising a specific organic peroxide represented by the following formula (2) and a polymerizable compound having a specific structure represented by the following formula (3) or (4), a thermopolymerizable composition having good storage stability and excellent curing property can be obtained. It is verified that this composition produces a solid polymer electrolyte even in the inside of an electrode or in the inside of a material, where active rays cannot reach, and the solid polymer electrolyte obtained has good adhesion to the electrode. 
(wherein the symbols have the same meanings as defined later).
A solid polymer electrolyte obtained starting from a urethane (meth)acrylate compound, which has been previously proposed by the present inventors (JP-A-9-73907) exhibits excellent ionic conductivity at room temperature and also at low temperatures and when this is used as the above-described polymerizable compound, the composition and in turn the solid polymer electrolyte can exhibit excellent ionic conductivity at room temperature and also at low temperatures. This is also verified.
More specifically, the present invention relates to a thermopolymerizable composition described below, a solid polymer electrolyte obtained by polymerizing the thermopolymerizable composition, a battery and an electric double-layer capacitor each using the solid polymer electrolyte, and processes for manufacturing the same.
(1) A thermopolymerizable composition comprising at least one thermopolymerizable compound having a polymerizable functional group and capable of forming a polymer having a cross-linked and/or side chain structure by the polymerization, at least one electrolyte salt, at least one polymerization initiator and at least one polymerization retarder having a vinyl group.
(2) The thermopolymerizable composition as described in (1) above, wherein said polymerization retarder is a compound having a structure represented by the following formula (1): 
wherein A represents a linear, branched or cyclic alkylene group having from 1 to 20 carbon atoms, a linear, branched or cyclic alkenyl group having from 2 to 20 carbon atoms or a linear, branched or cyclic alkynyl group having from 2 to 20 carbon atoms; Ar represents an aryl group which may have a substituent; a represents an integer of 0 or 1; and two Ar groups may the same or different.
(3) The thermopolymerizable composition as described in (1) above, wherein said polymerization retarder is a compound having a structure represented by formula (1) where A is a linear, branched or cyclic alkylene group having from 1 to 20 carbon atoms; Ar is a phenyl group which may have a substituent; a is an integer of 0 or 1; and two Ar groups may be the same or different.
(4) The thermopolymerizable composition as described in (1) above, wherein said polymerization initiator is an organic peroxide represented by the following formula (2): 
wherein X represents a linear, branched or cyclic alkyl or alkoxy group which may have a substituent; Y represents a linear, branched or cyclic alkyl group which may have a substituent; and m and n each independently represents 0 or 1, provided that the combination of (m,n)=(0,1) is excluded.
(5) The thermopolymerizable composition as described in (1) above, wherein said polymerization initiator is an organic peroxide represented by formula (2) where X and Y each is a linear, branched or cyclic alkyl group which may have a substituent; and m and n each independently is 0 or 1, provided that the combination of (m,n)=(0,1) is excluded.
(6) The thermopolymerizable composition as described in (4) above, wherein said organic peroxide has an active oxygen amount of from about 1 to about 1,000 ppm based on the thermopolymerizable composition.
(7) The thermopolymerizable composition as described in (4) above, wherein said organic peroxide is selected from the group consisting of diacyl peroxides, peroxydicarbonates and peroxy esters each containing no benzene ring.
(8) The thermopolymerizable composition as described in (1) above, wherein said thermopolymerizable compound comprises a compound having an ethylenically unsaturated polymerizable group.
(9) The thermopolymerizable composition as described in (1) above, wherein said thermopolymerizable compound comprises a polymerizable compound having either one of the polymerizable functional groups represented by the following formulae (3) and/or (4): 
wherein R1 and R3 each represents hydrogen atom or an alkyl group; R2 and R5 each represents a divalent group containing oxyalkylene, fluorocarbon, oxyfluorocarbon or a carbonate group; R4 represents a divalent group having 10 or less carbon atoms; provided that R2, R4 and R5 each may contain a hetero atom and may have any of linear, branched and cyclic structures; x represents 0 or an integer of from 1 to 10; provided that when a plurality of polymerizable functional groups represented by formula (3) or (4) are contained in the same molecule, R1, R2, R3, R4, R5 or x in each polymerizable functional group may be the same or different.
(10) The thermopolymerizable composition as described in (9) above, which further comprises at least one organic solvent selected from the group consisting of carbonic esters, aliphatic esters, ethers, lactones, sulfoxides and amides.
(11) The thermopolymerizable composition as described in (1) above, wherein the content of said organic solvent is from about 300 to about 1,500 wt % based on the thermopolymerizable compound.
(12) The thermopolymerizable composition as described in (1) above, which comprises at least one kind of inorganic fine particle having an average particle size of from about 0.005 to about 100 xcexcm.
(13) The thermopolymerizable composition as described in (1) above, wherein said electrolyte salt is at least one selected from the group consisting of alkali metal salts, quaternary ammonium salts, quaternary phosphonium salts, transition metal salts and protonic acids.
(14) The thermopolymerizable composition as described in (13) above, wherein at least one electrolyte salt is selected from the group consisting of LiPF6, LiBF4, LiAsF6 and LiN(Rxe2x80x94SO2)2 (wherein R represents a perfluoro-alkyl group having from 1 to 10 carbon atoms).
(15) A solid polymer electrolyte obtained by thermopolymerizing a thermopolymerizable composition comprising at least one thermopolymerizable compound having a polymerizable functional group and capable of forming a polymer having a cross-linked and/or side chain structure by the polymerization, at least one electrolyte salt, at least one polymerization initiator and at least one polymerization retarder having a vinyl group.
(16) An electrode for batteries or electric double-layer capacitors, comprising an electroactive substance or a polarizable material and a solid polymer electrolyte obtained by thermopolymerizing a thermopolymerizable composition comprising at least one thermopolymerizable compound having a polymerizable functional group and capable of forming a polymer having a cross-linked and/or side chain structure by the polymerization, at least one electrolyte salt, at least one polymerization initiator and at least one polymerization retarder having a vinyl group.
(17) A battery using a solid polymer electrolyte obtained by thermopolymerizing a thermopolymerizable composition comprising at least one thermopolymerizable compound having a polymerizable functional group and capable of forming a polymer having a cross-linked and/or side chain structure by the polymerization, at least one electrolyte salt, at least one polymerization initiator and at least one polymerization retarder having a vinyl group.
(18) The battery as described in (17) above, wherein at least one material selected from the group consisting of lithium, lithium alloys, carbon materials capable of occluding or releasing lithium ion, inorganic compounds capable of occluding or releasing lithium ion and electrically conducting polymers capable of occluding or releasing lithium ion is used as the negative electrode of the battery.
(19) The battery as described in (17) above, wherein at least one material selected from the group consisting of electrically conducting polymers, metal oxides, metal sulfides and carbon materials is used as the positive electrode of the battery.
(20) An electric double-layer capacitor comprising a solid polymer electrolyte obtained by thermopolymerizing a thermopolymerizable composition comprising at least one thermopolymerizable compound having a polymerizable functional group and capable of forming a polymer having a cross-linked and/or side chain structure by the polymerization, at least one electrolyte salt, at least one polymerization initiator and at least one polymerization retarder having a vinyl group.
(21) A process for manufacturing a battery, comprising placing a thermopolymerizable composition comprising at least one thermopolymerizable compound having a polymerizable functional group and capable of forming a polymer having a cross-linked and/or side chain structure by the polymerization, at least one electrolyte salt, at least one polymerization initiator and at least one polymerization retarder having a vinyl group, in a structure body for constructing a battery, or disposing the composition on a support, and then heat-curing the thermopolymerizable composition.
(22) A process for manufacturing an electric double-layer capacitor, comprising placing a thermopolymerizable composition comprising at least one thermopolymerizable compound having a polymerizable functional group and capable of forming a polymer having a cross-linked and/or side chain structure by the polymerization, at least one electrolyte salt, at least one polymerization initiator and at least one polymerization retarder having a vinyl group, in a structure body for constructing an electric double-layer capacitor, or disposing the composition on a support, and then heat-curing the thermopolymerizable composition.